Centennial-scale environmental changes in Terhiin Tsagaan Lake, Mongolia inferred from lacustrine sediment: Preliminary results

Terhiin Tsagaan Lake in central Mongolia is a dammed lake formed by the eruption of a volcano at ca 7,000 year BP. Eight short sediment cores, lake water samples and inlet and outlet samples were collected. The concentrations of major dissolved species in the samples revealed that the lake water is Ca²⁺-HCO³⁻ type, but that the Ca²⁺ and HCO₃₋ levels are relatively low. The water has high dissolved SiO₂ concentrations, which suggests favorable conditions for formation of biogenic silica depending on the surrounding environment. Analyses of the X-ray CT images of the cores showed that the average brightness over the horizontal direction, which represents averaged density of each layer, exhibited pulsed-like peaks as well as gradual fluctuations. The pulsed-like peaks in the pattern were probably resulted from the density currents including clasitic materials from the catchment area, therefore, can be used for the indicator for some hydro-climatic events and/or lake level changes. The gradual fluctuation of the averaged brightness was well correlated with the contents of biogenic silica plus organic matter. The profiles of the brightness can provide high resolution information regarding the biological activities in lake. The patterns of averaged brightness of five cores, obtained from deeper parts of the lake, exhibited similar patterns, with a positive peak in upper part of the sediments, and the minimum brightness just below the positive peak. The comparison of the core obtained from the previous study (Sevastyanov et al. 1989) showed that the short cores obtained from the deeper parts of the lake in present study can provide a record back to 1,000 year B.P. The minimum brightness indicates that the higher biogenic response at 200 years ago. The presence of a positive peak above the minima indicates that the condition became lower biogenic response at 100 years ago and then gradual increase in response as time progressed toward the present.